A magneto-optical disk has a memory layer for information. The memory layer is formed on a substrate by a perpendicularly magnetized film of a magnetic substance. Recording and reproduction of information on such a magneto-optical disk are carried out as follows. When recording, first, initialization is performed by orienting the magnetization direction of the memory layer in one direction (upward or downward direction) with a strong external magnetic field. Then, laser light is irradiated on an area in which information is to be recorded so as to heat the area to have a temperature not lower than the vicinity of the Curie point of the memory layer or the vicinity of the compensation point thereof. In this arrangement, the coercive force in the area is made zero or substantially zero, and then an external magnetic field (bias magnetic filed) whose direction is opposite to that of the magnetic field used for the initialization is applied to reverse the magnetization direction. When the irradiation of laser light is stopped, the temperature of the memory layer returns to room temperature, and therefore the reversed magnetization is fixed. As a result, information is thermomagnetically recorded.
In reproduction, the disk is irradiated with linearly polarized laser light, and information is optically readout by using phenomena (Kerr magnetic effect and Faraday magnetic effect) in which the plane of polarization of reflected light or transmitted light from the disk is rotated according to the magnetization direction of the memory layer.
On the other hand, magneto-optical disks on which information was recorded by the above-mentioned magneto-optical recording method have been focused as rewritable large-capacity memory elements. In order to achieve a magneto-optical disk which permits rewriting of information by performing initialization with a relatively weak initializing magnetic field and modulating the intensity of light while applying a recording magnetic field, i.e., a so-called overwritable magneto-optical disk by light-intensity modulation, it has been conventionally proposed to form a memory layer by exchange-coupled two-layer films.
Furthermore, as illustrated in FIG. 14, a magneto-optical disk disclosed in Japanese Publication for Examined Patent Application (Tokukohei) No. 5-22303 includes three magnetic layers, first to third magnetic layers 21 to 23, so as to reduce the initializing magnetic field and improve the stability of recording bits. The first magnetic layer 21 as a memory layer and the third magnetic layer 23 as a writing layer show perpendicular magnetization within temperature ranges between room temperature and their Curie points. On the other hand, the second magnetic layer 22 as an intermediate layer formed between the layers 21 and 23 exhibits in-plane magnetization at room temperature and perpendicular magnetization as the temperature is increased. As illustrated in FIG. 15, the third magnetic layer 23 is formed so that a coercive force H.sub.L thereof at room temperature is smaller than a coercive force H.sub.H of the first magnetic layer 21 and a Curie point T.sub.H thereof is higher than a Curie point T.sub.L of the first magnetic layer 21. Although not shown in the drawings, the second magnetic layer 22 is formed so that a Curie point T.sub.M thereof is located between the Curie points T.sub.L of the first magnetic layer 21 and the Curie point T.sub.H of the third magnetic layer 23.
The procedure for overwriting a magneto-optical disk having the above-mentioned structure is briefly explained below. An initializing magnetic field Hinit whose strength at room temperature is between the coercive forces H.sub.H and H.sub.L of the first and third magnetic layers 21 and 23 is applied as illustrated in FIG. 14. At this time, the magnetization direction of the first magnetic layer 21 remains unchanged, while the magnetization direction of the third magnetic layer 23 is aligned in one direction along the direction of the initializing magnetic field Hinit. In FIG. 14, the arrows shown in the magnetic layers 21 to 23 indicate the magnetization direction of the sub-lattice of the transition metal of each of the layers 21 to 23.
At this time, since the second magnetic layer 22 exhibits in-plane magnetization at room temperature, magnetic coupling forces (exchange forces) between the first magnetic layer 21 and the third magnetic layer 23 are prevented. As a result, the strength of the initializing magnetic field Hinit is further decreased. It is thus possible to align the magnetization direction of the third magnetic layer 23 in one direction.
Next, laser light whose intensity is modulated between high level I and low level II depending on information to be recorded is irradiated while applying a recording magnetic field H.sub.W whose strength is smaller than that of the initializing magnetic field Hinit and whose direction is opposite to that of the initializing magnetic field Hinit.
When laser light of high level I is irradiated, the temperature of the irradiated area exceeds the Curie points T.sub.L and T.sub.M of the first and second magnetic layers 21 and 22 and is raised to near the Curie point T.sub.H of the third magnetic layer 23. As a result, the magnetization direction of the third magnetic layer 23 is reversed along the direction of the recording magnetic field H.sub.W. Consequently, the magnetization direction of the third magnetic layer 23 is copied to the second magnetic layer 22 showing perpendicular magnetization due to exchange forces acting on the boundary thereof, and then copied to the first magnetic layer 21.
On the other hand, when laser light of low level II is irradiated, the temperature of the irradiated area is heated only to a temperature near the Curie point T.sub.L of the first magnetic layer 21. At this time, since the coercive force of the third magnetic layer 23 is larger than the recording magnetic field H.sub.W, the magnetization direction is not reversed, thereby maintaining the magnetization direction produced by the initialization. Similarly, the magnetization direction of the third magnetic layer 23 is copied to the first magnetic layer 21 through the second magnetic layer 22 by the exchange forces acting on the boundary thereof with a decrease of the temperature to room temperature.
In the above-mentioned procedure, new information corresponding to the intensity-modulated laser light is recorded on the first magnetic layer 21. Reproduction of the recorded information is carried out by irradiating laser light of an intensity lower than low level II.
However, the above-mentioned magneto-optical disk is configured so that the Curie point T.sub.M of the second magnetic layer 22 showing a transition from in-plane magnetization to perpendicular magnetization with an increase of the temperature and that the Curie points T.sub.H and T.sub.L of the first and third magnetic layers 21 and 23 to establish the relation T.sub.L &lt; T.sub.M &lt; T.sub.H. Therefore, if T.sub.M is close to T.sub.H, overwriting with light-intensity modulation cannot be performed smoothly.
Namely, in order to improve the characteristics of reproduced signals by increasing the Kerr rotation angle when irradiating linearly polarized laser light in reproduction, it is effective to use a material having a high Curie point T.sub.L for the first magnetic layer 21. In this case, if the second magnetic layer 22 is formed so as to satisfy the above-mentioned relation, the Curie point T.sub.M thereof becomes closer to the Curie point T.sub.H of the third magnetic layer 23. Actually, the magneto-optical disk disclosed in the above-mentioned publication has a structure in which, for example, when the Curie point T.sub.H of the third magnetic layer 23 is 180.degree. C., the Curie point T.sub.M of the second magnetic layer 22 is 170.degree. C. (see Japanese Publication for Examined Patent Application (Tokukohei) No. 5-22303, column 9, line 42 to column 10, line 12).
As described above, in the structure where T.sub.H and T.sub.M are close to each other, when laser light of high level I is irradiated to raise the temperature to near T.sub.H, it is necessary to reverse the magnetization direction of the third magnetic layer 23 along the direction of the recording magnetic field H.sub.W at a temperature not higher than the Curie point T.sub.M of the second magnetic layer 22 due to, for example, variations in the raised temperature caused by changes in ambient temperature. At this time, since the second magnetic layer 22 exhibits perpendicular magnetization, exchange forces from the second magnetic layer 22 act on the third magnetic layer 23. Therefore, with the recording magnetic field which is determined by only considering the coercive force of the third magnetic layer 23, the magnetization direction of the third magnetic layer 23 may not be satisfactorily reversed. Consequently, as mentioned above, overwriting with light-intensity modulation cannot be smoothly performed.